Switch having an arc stabilizing electrode



Aug. 30, 1966 J. N. CHUBB SWITCH HAVING AN ARC STABILIZING ELECTRODE Filed April 19, 1965 United States Patent 3,270,172 SWITCH HAVING AN ARC STABILIZING ELECTRODE John N. Chubb, Didcot, England, assignor to The English Electric Company Limited, London, England, a British company Filed Apr. 19, 1965, Ser. No. 449,246 Claims priority, application Great Britain, June 30, 1961, 23,647/ 61 2 Claims. (Cl. 200-144) This application is a continuation-in-part application of copending application Serial No. 206,030, filed June 28, 1962.

This invention relates to vacuum electric switches having electrodes between which arcing can take place. The invention is particularly concerned with switches in which these switch electrodes can be moved into and out of contact with one another and therefore they will be referred to hereinafter, where appropriate, as switch contacts.

Before a vacuum electric switch can be put into normal operation it is necessary for it first to be current conditioned. The process of current conditioning is carried out in order to remove contaminating layers from the regions of the switch electrode surfaces which are accessible for arcing. If this were not done, the gas evolved from the contaminating layers by arcing during normal operation would cause the vacuum to decay and give rise to malfunction of the switch. The current conditioning is performed during the vacuum processing of the switch structure after the gas pressure in the envelope has been reduced, possibly with the aid of baking, to between 10- millimetres of mercury. The conditioning process consists of repeatedly drawing the are between the main switch electrodes, starting at a current of about 50 amps and increasing progressively to a few hundred amps as the pressure pulses associated with each arcing period diminish with the removal of the contaminating layers from the switch electrodes. The duration of the initial arcing periods may be 10 milliseconds, and as the contamination is removed the arcing duration may be increased along with the arcing current to achieve the most rapid and effective conditioning of the switch electrodes. The ultimate pressure in the switch after full vacuum processing and conditioning may be between 10- and 10' millimetres of mercury.

The current in a vacuum arc discharge may be carried by a multiplicity of individual and separate arc channels, each channel typically carrying between 100 and 200 amperes. During arcing, the arcing channels, which strike initially over the centre portion of the switch electrodes repel one another and consequently tend to move outwardly. At low currents this outward movement may be slight, but as the full capacity current of the switch is approached, the outward movement causes the arc channels to extend out towards the furthermost edges of the electrodes.

To avoidevolution of gas at the highest currents to which the switch may be subjected it is necessary that the whole of the surfaces of the electrodes on which the arc may strike have been current conditioned. Present day requirements call for vacuum switches having an interruption capacity in excess of 10,000 amps. To current condition the whole of the electrode surfaces available for arcing at the full interrupting capacity of the switch using conventional switch electrode designs and conditioning procedures would require conditioning currents comparable to the switch interrupting capacity. Conditioning at these currents would be uneconomical and would seriously reduce the switch electrode life.

Accordingly, it is the object of the present invention 3,270,172 Patented August 30, 1966 to provide means, in a vacuum electric switch, to limit the surface area of the switch electrodes over which the arc can spread in such a way that the whole surface area of the electrodes which is available for arcing at any current up to the full interrupting capacity of the switch, can be fully current conditioned using conditioning currents of only a few hundred amps.

A number of embodiments of vacuum electric switch in accordance with the invention will now be particularly described, by way of example only, with reference to the accompanying drawings in which:

FIGURES l and 2 are diagrammatic sections throug one embodiment of vacuum switch, details of the casing which form no part of the invention having been omitted. The contacts are shown open in FIGURE 1 and closed in FIGURE 2.

FIGURE 3 is a diagrammatic section through a second embodiment of vacuum switch, the contacts being shown open.

As shown in the embodiment of FIGURES 1 and 2, the switch comprises two butt contacts 10, 11 the contact 10 being stationary and the contact 11 movable towards or away from the contact 10. The contacts have flanged heads 10a, 11a, which are formed with flat circular contact surfaces 10b, 11b. Outwardly of these surfaces the heads are chamfered to form two annular surfaces 100, 110 of truncated conical shape which are directed away from one another. Thus when the contact surfaces 10b, 11b are in abutment, an annular space 12 is left between the surfaces 100, 110, which is of wedge shaped cross-section diverging in the radially outward direction.

Surrounding, but spaced from, the heads of the two contacts is a stabilizing electrode 13 of cylindrical shape having a fiat annular web 13a integral therewith and projecting from the inner wall of the cylinder into the space between the surfaces 100, 11c.

The second embodiment of vacuum switch shown in FIGURE 3' comprises an outer ring contact 16 and an inner solid cylindrical contact 17, the outer contact being supported on an actuating rod 18 movable along its length in either direction to cause the outer contact 16 to move into or out of abutment with the inner contact 17.

The inner wall of the outer contact 16 has a cylindrical 1 contact surface 16b and frusto-conical surfaces 160 which are inclined or flared outwardly from opposite ends of the surface 16b. Likewise the inner contact 17 has a cylindrical contact surface 17b for engagement with the surface 161) and frusto-conical surfaces which are inclined or tapered towards the axis of contact 17 from opposite ends of the surface 17b. Thus two annular spaces 19 are formed between opposed pairs of surfaces 160, 170, the spaces being of wedge-shaped crosssection. Adjacent opposite ends of the contacts 16, 17 are disc shaped stabilizing electrodes 20, 20' located normal to the axis of the contact 17, each having an annular flange 20a integral therewith and coaxial with the axis of the contact 17. Each flange 20a projects from the disc into the adjacent space 19 between the surfaces 16c, 170 of the contacts. The electrode 20' has a central opening through which extends a rod 21 in spaced relation thereto, the rod supporting the contact 17.

There are two aspects of the behaviour of vacuum arcs which are relevant to the operation of the present invention.

First let us consider a vacuum arc discharge maintained between two electrodes of the same material. The voltage at which the discharge is maintained is observed to be a characteristic of the electrode material and is nearly independent of the current carried, for example arcing voltages of 21 volts, 24 volts and 26 volts are observed respectively for copper, molybdenum and tungsten electrodes; and in the case of copper this discharge voltage may perhaps increase by 40% as the discharge current increases from a few hundred amps to several kilo amps.

Secondly, each channel of the vacuum arc discharge takes a conical from with the apex located at a small luminous spot on the cathode and the current in this cone tends to be maintained in a line of sight geometry so that a discharge between the two electrodes will prefer to avoid any non-line of sight path between the electrode surfaces. The manner in which these two aspects of vacuum arc behaviour may be employed to achieve restriction of the area available for arcing of the two cooperating vacuum switch electrodes may be understood with respect to the diagrams shown in FIGURES 1 and 2.

FIGURE 2 shows two cooperating electrodes made of the same material whose outer surfaces have interposed between them an annular web 13a in a stabilizing electrode 13, which is made from the same material as the two electrodes. On separation of the two electrodes arcing is initiated between the two cooperating surfaces b and 11b. At high operating currents the arc channels will tend to move away from these cooperating surfaces towards the outer conical surfaces of the electrodes 10c and 11c. The aperture in the annular web acts on the individual arc channels so as to prevent movement beyond the region specified by this aperture. Movement beyond the limit of the aperture would require two are channels to be maintained in series if line of sight geom etry of individual channels is to be maintained, that is to say one channel between surface 10c and the annular Web 13a and a second channel between annular web 13a and surface 110.

Since each of two such are channels would require similar discharge voltage to any of the arc channels between the two main electrode surfaces within the limit of the aperture of the annular web, existence of the series are channels is energetically inadmissible, because the discharge voltage necessary would be twice that for areing within the aperture.

The foregoing embodiments of the present invention have been considered with the two main electrodes and the stabilizing electrode all made of the same material. However, the mechanism of stabilization described is also applicable for combinations of electrode material satisfying the basic requirement that the voltages of the line of sight discharge between the two main electrodes shall be less than the voltage across any series connection of discharge paths, including the stabilizer electrode. If, for example, the two main electrodes are made of one type of material and the stabilizer is made of a second, different, material then the above requirement is that the sum of the discharge voltages between the first and second materials and between the second and first materials shall be greater than the discharge voltage between the two electrodes of the first material. If again, the two main electrodes are made of two different materials, the first and second materials, and the stabilizer is made of a third material, then the requirement of stabilization will be satisfied for arcing in one direction if the discharge voltages between first and third, and third and second materials is greater than between the first and second materials, and for arcing in the reverse direction that the discharge voltage between the second and third and between the third and first shall be greater than between the second and first materials.

Even though no arcing takes place to the stabilizing electrode, this electrode may take some part in the maintenance of the vacuum environment between the two main switch electrodes. The stabilizing electrode will be subjected to heating from radiation from nearby discharges and also to bombardment by energetic particles from the main arcing electrodes. It may therefore be advantageous for this stabilizing electrode or electrodes, to be made from a getter material such as titanium, zirconium or tantalum so that such heating and bombardment may lead to absorption of any small quantities of gasses liberated from the main electrodes during normal operation of the switch.

It may also be desirable to current condition the surface of the stabilizing electrode, and for this purpose an electrical connection from the stabilizing electrode to the outside of the vacuum switch can be provided so that arcing between the stabilizing electrode and either contact may be deliberately initiated during the conditioning process.

I claim:

-1. In a vacuum electric switch,

a cylindrical inner electrode,

a cylindrical outer electrode surrounding said inner electrode,

said inner electrode having an outer surface comprising a first surface portion of constant diameter along its length and second and third surface portions extending from opposite ends of said first surface portions and each having, at all positions along its length, a diameter less than that of the first portion,

said outer electrode having an inner surface comprising a first surface portion of constant diameter along its length and disposed opposite the first surface portion of the inner electrode, and second and third surface portions, extending from opposite ends of the first surface portion, each having, at all positions along its length, a diameter greater than that of the first portion, said second and third surface portions lying opposite the second and third surface portions respectively of the inner electrode,

a first cylindrical stabilising electrode lying between, but spaced from, the second surface portions of the inner and outer switch electrodes,

at second cylindrical stabilising electrode lying between, but spaced from, the third surface portions of the inner and outer switch electrodes,

said stabilising electrodes being made of a material satisfying the condition that the voltages of the line of sight discharge between the portions of constant diameters shall be less than the voltage across any series connection of discharge paths including either stabilising electrode, and means for moving one said switch electrode into and out of contact with the other switch electrode. v

2. A vacuum electric switch according to claim 1 wherein the surface portions of the inner and outer electrodes which are exposed to one another have been current conditioned by drawing an arc therebetween.

References Cited by the Examiner UNITED STATES PATENTS 1,784,302 12/1930 Millikan et a1. 200-144 7,996,304 4/1935 Millikan et al. 200--144 2,900,476 8/ 1959 Reece 200-144 2,976,382 3/1961 Lee 200-444 3,014,110 12/1961 Cobine 200-144 FOREIGN PATENTS 389,463 3/ 1933 Great Britain. 571,959 1/ 1958 Italy.

ROBERT K. SCHAEFER, Primary Examiner. ROBERT S. MACON, Examiner. 

1. IN A VACUUM ELECTRIC SWITCH, A CYLINDRICAL INNER ELECTRODE, A CYLINDRICAL OUTER ELECTRODE SURROUNDING SAID INNER ELECTRODE, SAID INNER ELECTRODE HAVING AN OUTER SURFACE COMPRISING A FIRST SURFACE PORTION OF CONSTANT DIAMETER ALONG ITS LENGTH AND SECOND AND THIRD SURFACE PORTIONS EXTENDING FROM OPPOSITE ENDS OF SAID FIRST SURFACE PORTIONS AND EACH HAVING, AT ALL POSITIONS ALONG ITS LENGTH, A DIAMETER LESS THAN THAT OF THE FIRST PORTION, SAID OUTER ELECTRODE HAVING AN INNER SURFACE COMPRISING A FIRST SURFACE PORTION OF CONSTANT DIAMETER ALONG ITS LENGTH AND DISPOSED OPPOSITE THE FIRST SURFACE PORTION OF THE INNER ELECTRODE, AND SECOND AND THIRD SURFACE PORTIONS, EXTENDING FROM OPPOSITE ENDS OF THE FIRST SURFACE PORTION, EACH HAVING, AT ALL POSITIONS ALONG ITS LENGTH, A DIAMETER GREATER THAN THAT OF THE FIRST PORTION, SAID SECOND AND THIRD SURFACE PORTIONS LYING OPPOSITE THE SECOND AND THIRD SURFACE PORTIONS RESPECTIVELY OF THE INNER ELECTRODE. 